A deicer provided on a windshield of an automobile requires a high electric power of 1500 W which needs a charge current at 70 V, for example. In order to operate the high voltage load, the conventional type apparatus employs a switch 41 within a line 3 which connects an alternator 1 to a battery 2 as shown in FIG. 9, so that the charge current from the alternator 1 is switched from the battery voltage level to the high voltage load 5 level. Since the voltage level of the battery 2 is reduced while the switch 41 is connected to the high voltage load 5, the alternator 1 is fully excited so that high voltage such as 70 V is supplied to the high voltage load 5.
The voltage of the battery 2 is returned as a feedback signal to the alternator 1 through a key switch 7 and a voltage regulating circuit 6 in order to control the output of the alternator 1 for maintaining the battery voltage within the predetermined value. Since the high voltage load such as a front deicer is used during the starting period of the automobile when the battery is discharged for starting the engine, and since the conventional type apparatus could not charge the battery while the high voltage load is operated, the conventional type apparatus has such a disadvantage that the battery is too discharged to keep the battery voltage level.